A developing device which develops an electrostatic latent image with a two-component developer including carrier and toner is widely used in copying machines as electrophotographic apparatuses. When developing the image, the toner is consumed, while the carrier is not consumed and remains in the developing device. In the developing device, carrier and toner are continually mixed. This causes a resin coating layer to be separated from the carrier surface and the toner to adhere to the carrier surface. As a result, the charging rate of the developer gradually decreases as illustrated in the curved line (b) of FIG. 39. In order to prevent the decrease in the charging rate, the carrier is usually replaced periodically.
The charging rate of the carrier just before replacement is far below that of a newly replaced carrier. Thus, with the method which requires periodical replacement of the carrier, the image quality varies significantly before and after the replacement of the carrier. In addition, replacing the carrier periodically by removing old carrier from and filling the developing device with new carrier is a time consuming task and unhygienic as the toner spills from the developing device and rises. To solve such a problem, a trickle-type developing device which gradually discharges the carrier from the developing device and supplies new carrier to the developing device is proposed.
Typical trickle-type developing devices are disclosed, for example, in Japanese Publication for Examined Patent Application No. 21591/1990, Japanese Publication for Examined Utility Model Application No. 2596/1985, and Japanese Publication for Unexamined Patent Application No. 267683/1989. With such a developing device, a new developer is gradually supplied from a developer supply to the developing device. When the amount of the developer in the developing device increases excessively, the old developer overflows from the developer discharge opening into a waste container.
With such a developing device, new developer is gradually added, while old developer is gradually discharged. Therefore, as illustrated in the curved line (a) of FIG. 39, the charging rate of the developer becomes substantially uniform after it has been used for a predetermined period of time. However, the charging rate of the developer decreases to a certain extent until it will have been used for a predetermined time after the addition of the new developer to the developing device. Another advantage of this developing device is that the waste container is needed to be replaced only when the waste container is fill up with the old developer. Namely, it is not necessary to replace the carrier periodically according to the deterioration of the carrier. Moreover, since only the waste container is replaced, a reduced amount of toner rises during replacement.
In a clamshell-type copying machine, the sheet transport path is exposed by moving the upper body of the copying machine round the supporting point. The supporting point is located on an end of the copying machine, adjacent to the developer discharge opening of the developing device (see, for example, FIG. 1 of Japanese Publication for Unexamined Patent Application No. 267683/1989). On the other hand, in the conventional trickle-type developing device, the developer discharge opening is always open and ready to discharge the used developer. With this structure, for example, if the developer tilts as a result of opening the upper body of the copying machine to remove a jammed sheet, the developer in the developing device moves toward the developer discharge opening. This causes the developer to be unnecessarily discharged from the developer discharge opening.
Moreover, the conventional structure is designed without fully considering the handling of the developer when loading/unloading the developing device and when discarding the developer. Therefore, there is room for improvement.
Furthermore, with the conventional structure, excessive developer is discharged from the developer discharge opening into the waste container when, for example, a movement of the copying machine gives an impact on the developing device.